This invention relates generally to improvements in fuel cell operability during conditions where moisture-prone components are exposed to temperatures where water may freeze, and more particularly to a fuel cell system and method for selectively routing a cathode slip stream to reduce the likelihood of frozen water blockage of the fuel cell during cold weather starts.
Fuel cells convert a fuel into usable electricity via chemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) and related power-generating sources. In a typical fuel cell (such as a proton exchange membrane or polymer electrolyte membrane—in either event, PEM—fuel cell), a pair of catalyzed electrodes are separated by an ion-transmissive medium in the form of a polysulfonated membrane (such as Nafion™) such that an electrochemical reaction may occur when an ionized form of a reducing agent (such as hydrogen, H2) introduced through one of the electrodes (the anode) crosses the ion-transmissive medium and combines with an ionized form of an oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode). Upon combination at the cathode, the ionized hydrogen and oxygen form water. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load. The flow of this DC energy is the basis for power generation by the fuel cell.
The PEM fuel cell stack needs to run during varying ambient environmental conditions, including those that are cold, wet or both. Left unchecked, such conditions may hamper effective fuel cell startup and shutdown. For example, during shutdown, a certain amount of water (much of which may have been generated during operation of the fuel cell system) has to be removed to ensure that ice blockage of key flowpaths is avoided and that a subsequent startup, warm-up, and drive-away is still possible even after the system has been exposed to freezing conditions. Removing water from the fuel cell's anode loop is especially difficult as it doesn't have the high gas volume and flow velocity that the cathode loop does as a way to purge any excess water. One way to facilitate anode loop water evacuation is by drawing the water directly through the ion-transmissive medium of the various fuel cells toward the cathode. Unfortunately, this is a fairly slow process (often taking over a minute to drop anode water content to an appreciable level). This approach can also lead to excessive membrane drying out, which may adversely impact the durability of the individual fuel cells.
Another way to reduce or eliminate the chance of such flowpath ice formation is to allow some of the hydrogen from the anode loop to be introduced into the cathode loop during fuel cell system shutdown and startup; such an approach may be effected through a valve placed between the anode and cathode loops and allowed to remain open long enough (possibly for only a few seconds) to promote the hydrogen flow. During the shutdown, the valve provides a quicker path for water to leave the anode instead of the slow method of drawing water through the ion-transmissive medium. During the startup, this catalytic reaction of hydrogen and oxygen (in addition to possibly helping reduce open circuit voltage (OCV)) produces heat that may be used to raise the temperature of adjacent flowpaths and components. While this approach is more capable of promoting prompt, efficient warmup of a fuel cell system that has been exposed to freezing conditions, the relatively large thermal mass of the valve itself makes it susceptible to ice formation and related blockage. Moreover, such valves typically include a flow-regulating opening (in the form of an orifice) that by virtue of its precisely known size is used to provide precise measurement or control functions. Unfortunately, the size and precision needed to establish its flow-regulating function also make the orifice particularly susceptible to the types of ice blockage associated with the remainder of the valve as discussed above.